The present invention relates to a fuel cell power plant incorporating a fuel cell of the molten carbonate type which operates with an electrolyte such as sodium carbonate, potassium carbonate or the like in the molten state, and more particularly to an improvement of the overall thermal efficiency of the fuel cell power plant.
In a fuel cell known as a molten carbonate fuel cell, a carbonate such as lithium carbonate, potassium carbonate or the like is used as an electrolyte, and this type of fuel cell operates at 600 to 700.degree. C., at which temperature the carbonate is molten. The molten carbonate type fuel cell exhibits a vigorous reaction because it operates at high temperatures; therefore, it does not require an expensive catalyst such as platnum. Also, the molten carbonate fuel cell can function with any of a wide selection of fuels, because it is not adversely affected by carbon monoxide which hinders the operation of the phosphate type fuel cell which has been developed recently. Also, this type of fuel is expected to operate at a thermal efficiency of 45% or higher, when used in combination with a coal gas process. For these reasons, the molten carbonate type fuel cell is attracting attention as a promising power generating sytem which is expected to be put into practical use in the next generation. In fact, this type of fuel cell has been called the fuel cell of the second age while the phosphate type fuel cell has been called the fuel cell of the first age.
In the operation of the molten carbonate fuel cell, hydrogen or a hydrogen-containing gas as a fuel or reactive gas is supplied to an anode, while a cathode is supplied with a mixture of air and carbon dioxide. The air and carbon dioxide supplied to the cathode receive electrons and produce carbonate ions which are introduced into an electrolyte. In the anode, the hydrogen reacts with the carbonate ions in the electrolyte and produces carbon dioxide and water, while emitting electrons. Thus, the molten carbonate fuel cell consumes carbon dioxide at its cathode and produces carbon dioxide and water at its anode. In the molten carbonate fuel cell, therefore, the reactive gas is diluted by carbon dioxide and water produced in the anode if the rate of utilization of the reactive gas, i.e., the ratio of the quantity of the reactive gas actually consumed in the anode to the quantity of reactive gas supplied to the anode, is increased. It is also known that an increase of the reactive gas utilization rate causes a drastic reduction in the concentration of the component of the reactive gas contributing to the reaction, hence a reduction in the output voltage and thermal efficiency of the fuel cell, because the reactive gas is consumed so quickly.
Thus, an increase in the fuel utilization rate reduces, as shown in FIG. 7, the efficiency of the cell expressed as (cell output power (KW).times.860)/(reaction heat of reactive gas actually consumed in anode), so that the efficiency of the cell decreases as the rate of utilization of the fuel increases. Cell efficiency shown in FIG. 7 is based on a rate of fuel utilization of 50%.
The efficiency of installed fuel cells is represented by cell power (KW).times.860/reaction heat of reactive gas supplied to the anode, and can be determined as (fuel cell efficiency) x (rate of utilization of reactive gas). To improve thermal efficiency of a fuel cell power plant in this configuration, attempts have been made to develop a fuel cell in which cell efficiency decreases only slightly even when the rate of utilization of the reactive gas increases, as well as a power generating system in which high thermal efficiency is obtained even at a reduced rate of fuel utilization by virtue of an increased heat recovery rate.
An example of a conventional fuel cell power plant with a heat recovery system is disclosed in "Fuel Cell Power Plant Integrated Systems Evaluation" of EPRI, January of 1981 (pages E-5 to E-9, particularly FIG. E-3). The fuel cell power plant shown in the FIG. E-3, includes a fuel cell, a turbine driving a compressor and a generator, a reformer for reforming fuel into a reactive gas, a combustor supplying the reformer with heat for effecting a reforming reaction, a heat recovery system and a drain separator for separating water from anode exhaust gas. The cathode of the fuel cell is supplied with a compressed air from a compressor and conbustion products from the combustor. A part of cathode exhaust gas is delivered to the turbine as a turbine driveing gas and drives it to thereby operate the compressor and the generator. Then the cathode gas is exhausted from the turbine. The turbine exhaust gas is sent to the waste heat recovery system. The remaining of the cathode exhaust gas is sent to the cathode as a cathode recirculated gas. The anode is supplied with a reactive gas, into which a fuel is reformed, after being heated by a heater or heat exchanger. Anode exhaust gas is fed to the combustor as a gas for combustion after passing through various heat exchangers and the drain separator (knockout drum), and is combusted under the presence of the compressed air from the compressor. The drain separator separates water from the anode exhaust gas. The water separated there is sent to the reformer in order to use it reforming the fuel after being heated by the heat recovery system and a heat exchanger.
In the fuel cell power described above, the temperature of the turbine driving gas, i.e., the cathode exhaust gas, ranges from 700.degree. to 750.degree. C., which is lower than the temperature of the gas supplied to the gas turbine which is ordinarily used. Therefore, the thermal efficiency of the turbine is comparatively low. The temperature of the turbine exhaust gas is as low as 300.degree. to 400.degree. C. Also, the steam generated in the waste heat recovery system has to be supplied as reforming steam to the reformer, so that there is not sufficient steam to drive a steam turbine if the turbine is provided for recovering heat. It is, therefore, quite difficult to recover the heat by means of a steam turbine.
Thus, in the described fuel cell power plant, the temperature of the turbine driving gas is so low that the thermal efficiency of the waste heat recovery system is inevitably low as compared with the gas turbine combination plant (gas turbine+steam turbine) which has been put into practical use. This undesirably impedes the improvement of the overall thermal efficiency of the fuel cell power plant, which is defined as follows:
overall efficiency of whole plant=electric ouput (fuel cell electric output+gas turbine electric output+steam turbine electric output) (KW).times.860/ (fuel input heat (Kcal)/H) .
There is a conventional fuel cell power plant in which a coal gasification system and a steam turbine system are combined with the fuel cell power plant. In this plant, the anode of a fuel cell is supplied with a reactive gas which is formed by refining a crude gas made by gasification of a coal in a coal gasification system. Anode exhaust gas is burned with an air compressed by a compressor and the resultant combustion gas is sent to the cathode of the fuel cell. A part of the cathode exhaust gas is sent to an expansion turbine which drives the compressor and a generator. The turbine exhaust gas heats a condensate from the steam turbine system in the wast heat recovery system. A part of the heated condensate is sent to the coal gasification system and is evaporated in steam which is sent to a steam turbine after being superheated by the cathode exhaust gas, while the remaining is sent to the coal gasification system and then sent directly to another steam turbine.
In this fuel cell power plant having a coal gasification system, since the temperature of the gas supplied to the expansion turbine for driving it is low, the thermal efficiency of the gas turbine combined plant utilizing the heat possessed by the gas which drives the expansion turbine is lowered to reduce the overall thermal efficiency of the plant, as in the case of the first-mentioned fuel cell power generating plant.
Japanese Patent Publication No. 56231/1983 proposes supplying combustion air to the exhaust gas coming from the combustion section of a reformer in a power generating plant utilizing a phosphate type fuel cell, such as to burn the exhaust gas to raise the temperature, the exhaust gas, having had its temperature raised, being then supplied to the expansion turbine, thereby elevating the expansion turbine inlet gas temperature. This method, when applied to a power generating plant employing a molten carbonate type fuel cell, however, presents some problems. Since the combustion gas from the reformer cannot be used as the carbon dioxide to be supplied to the cathode, it is necessary to find a way to supply carbon dioxide. For instance, a part of the anode gas is bypasses and is burned in a catalytic combustor or the like to become carbon dioxide which is to be fed to the cathode. When the gas burned in the combustor is supplied to the cathode, it is necessary to cool the hot combustion gas down to the cell operating temperature. In view of this fact, heating said gas in the combustor serves no purpose.
The temperature of the cathode exhaust which gas depends on the cell operating temperature, is low. Therefore, when the cathode exhaust gas is mixed with the combustor exhaust gas and supplied to the expansion turbine, it is not possible to raise the turbine inlet gas temperature to a sufficiently high level. Therefore, in order for the gas temperature at the turbine inlet to be sufficiently high, the cathode exhaust gas is not suppled to the expansion turbine so that heat cannot be recovered from the cathode exhaust gas. Consequently thermal efficiency is disadvantageously reduced. Furthermore, since the compressor outlet air is the air to be supplied to the cathode, the temperature supplied to the cathode is 200.degree. to 300.degree. C. lower than the operating temperature of the cell, so that an additional supply of the fuel is required to heat the air.
Another example of a conventional fuel cell power plant is disclosed in U.S. Pat. No. 3,976,507, whereas the fuel power plant is provided with a compressor for compressing air to be led to the cathode of the fuel cell, a turbine driven by the exhaust gas exhausted from the anode thereby to drive the compressor, a catalytic burner for burning the anode exhaust gas with air compressed by the compressor, and an autothermal reactor for reforming fuel by using the cathode exhaust gas. The anode exhaust gas is burned by the catalytic burner, using the compressed air which has not passed through the cathode and is cool, and the combustion gas is led to the turbine to drive it.